What causes "索引越界" in krahets/hello-algo?

Validate `index >= 0 && index < list.size` before get(); never bound loops on capacity.

krahets/hello-algo · error · IndexError

索引越界

Error message

索引越界

What it means

Ruby IndexError raised by MyList#get. The guard `raise IndexError, "索引越界" if index < 0 || index >= size` rejects negative indices (which Ruby's Array would otherwise wrap) and indices at/above the live element count. The custom List enforces strict bounds because its backing Array is sized to @capacity, not @size, so unchecked reads could return nil-filled capacity slots.

Source

Thrown at codes/ruby/chapter_array_and_linkedlist/my_list.rb:23

=end

### 列表类 ###
class MyList
  attr_reader :size       # 获取列表长度（当前元素数量）
  attr_reader :capacity   # 获取列表容量

  ### 构造方法 ###
  def initialize
    @capacity = 10
    @size = 0
    @extend_ratio = 2
    @arr = Array.new(capacity)
  end

  ### 访问元素 ###
  def get(index)
    # 索引如果越界，则抛出异常，下同
    raise IndexError, "索引越界" if index < 0 || index >= size
    @arr[index]
  end

  ### 访问元素 ###
  def set(index, num)
    raise IndexError, "索引越界" if index < 0 || index >= size
    @arr[index] = num
  end

  ### 在尾部添加元素 ###
  def add(num)
    # 元素数量超出容量时，触发扩容机制
    extend_capacity if size == capacity
    @arr[size] = num

    # 更新元素数量
    @size += 1
  end

View on GitHub (pinned to 69932aed18)

Solutions

Validate `index >= 0 && index < list.size` before get(); never bound loops on capacity.
For negative-index semantics, translate `index += list.size` first and re-check the range.
Recompute indices after any mutating operation.

Example fix

# before
v = list.get(i)
# after
raise ArgumentError if i < 0 || i >= list.size
v = list.get(i)

Defensive patterns

Strategy: validation

Validate before calling

# call BEFORE get(index)
raise ArgumentError, "bad index" if index < 0 || index >= list.size
v = list.get(index)

Try / catch

begin
  v = list.get(i)
rescue IndexError => e
  v = nil   # or log and skip
end

Prevention

Bound indices on `size`, never on `capacity`.
Translate negative indices manually (`index += size`) if you need Array-style wrap, then re-validate.
Recompute indices after every mutating operation.

When it happens

Trigger: Call list.get(i) with i < 0 (Ruby normally allows negative indexing, but this List forbids it), or i >= @size (pointing into the unused capacity tail, or past the last live element). Also after remove() decremented @size while a stale index is reused.

Common situations: Passing -1 expecting the last element (Array semantics) and hitting the negative-index rejection; loops bounded by @capacity instead of size; reusing an index captured before a remove()/clear(); off-by-one `i <= size`.

Related errors

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/8f97340d62812744. Report an issue: GitHub.